The present invention relates to a gain controller and, more particularly, to a gain controller used in a mobile telephone receiving system.
FIG. 3 shows a prior art mobile telephone receiving system. The system comprises an antenna 1, a radio unit 2, a digital signal processor (hereinafter referred to as DSP) 30, a DSP controller 70, a digital-to-analog converter 8, a volume resistor 9, a receiver amplifier 10, and a receiver 11. The DSP 30 includes a channel decoder 40 and a speech decoder 50. In FIG. 3, digital data received through the antenna 1 and decoded by the radio unit 2 comprises speech data and control data for each frame having a predetermined time length (for instance 90 msec). The decoded data from the radio unit 2 is decoded for each frame in the channel decoder 40 of the DSP 30 and the speech data in the frame is decoded to obtain digital speech data for the speech decoder 50. The digital speech data obtained by the speech decoder 50 is converted into analog signal by the digital/analog converter 8, then the level thereof is adjusted by the volume resistor 9, and the digital speech data is amplified by the receiver amplifier 10 to reproduce the speech by the receiver 11, such as a speaker. DSP controller 70 controls the operation of the DSP 30.
In such a prior art structure, it is only the volume resistor 9 that can control the sound level in the receiving system.
Japanese Patent Laid-Open Publication No. 123130/1991 discloses a hand-free car telephone system, which has means for controlling the volume from a speaker according to the running speed of the car. FIG. 4 is a block diagram showing this system. Referring to FIG. 4, the car telephone system 18 includes a handset 17, a microphone 16 to be used in hand-free communication, a volume control means 13 for controlling the speaker volume, and a telephone control means 12 for controlling the above car telephone system components. Car telephone system radio means 2 connects the car telephone system 18 with a base station (not shown in FIG. 4) via a radio line. During communication, a signal corresponding to the car running speed is supplied from speed detection means 14 to the volume control means 13 to control the volume from the speaker 15. Thus, the driver can continue communication without need of controlling the volume irrespective of ambient noise variations with running speed changes.